A common rail is conventionally used to distribute fuel to each of multiple fuel injection valves, which inject fuel into combustion chambers of an internal combustion engine. The common rail is provided with a pressure reducing valve, which decreases fuel pressure in the rail (referred to as rail pressure below) by discharging the fuel in the rail. JP 2006-242091A discloses one example of a pressure reducing valve, which includes an attachment part attached to a common rail, a valve body for opening and closing a discharge passage formed in the attachment part, a spring member for applying spring force to the valve body in a valve-closing direction and an electromagnetic coil for applying electromagnetic force to the valve body in a valve-opening direction. The valve body is arranged to be in a state to receive the rail pressure in the valve-opening direction.
The pressure reducing valve configured as described above is a normally-closed type. In this type of valve, when current supply to the electromagnetic coil is turned off, the valve body closes a fuel passage by the spring force against the biasing force of the rail pressure (referred to as fuel-generated valve-opening force), which biases the valve body in the valve-opening direction. When the current supply is turned on, the valve body opens the fuel passage against the spring force.
For this reason, the spring force need be larger than the fuel-generated valve-opening force to close the valve when the current supply is turned off. The electromagnetic force need be larger than force, which is determined by subtracting the fuel-generated valve-opening force from the spring force.
In a recent internal combustion engine, the rail pressure is increased to decrease exhaust emissions and improve fuel economy. In case that the rail pressure is increased, the fuel-generated valve-opening force is increased and the spring member is required to have large spring force.
It is however a general practice to variably control the rail pressure in accordance with operation states of the internal combustion engine, for example, to decrease the rail pressure when the internal combustion engine is in an idle operation state. That is, the rail pressure is not always controlled to a maximum pressure. For this reason, in case that the rail pressure is controlled to be low, the fuel-generated valve-opening force is decreased and correspondingly the electromagnetic force required to open the pressure reducing valve against the spring force need be increased. As a result, in case that the spring member having large biasing force is used, the fuel-generated valve-opening force is insufficient and the pressure reducing valve does not open even when the current is supplied under a low rail pressure state.
It is not possible to solve this problem by simply extending a period of current supply to the electromagnetic valve. Although it is possible to increase the electromagnetic force to be sufficiently large by increasing the number of turns of a winding of an electromagnetic coil, the pressure reducing valve becomes upsized. It is also possible to increase the electromagnetic force to be sufficiently large by using a high magnetic material for the electromagnetic coil. However, material cost becomes high.
Although the pressure reducing valve disclosed in JP 2006-242091A is attached to the common rail, the same problem arises in any valve devices other than the common rail as far as the pressure reducing valve is the normally-closed type and attached to the accumulator, in which fluid is accumulated.